crypto: fix Node_SignFinal

PR #11705 switched Node away from using using OpenSSL's legacy EVP_Sign*
and EVP_Verify* APIs. Instead, it computes a hash normally via
EVP_Digest* and then uses EVP_PKEY_sign and EVP_PKEY_verify to verify
the hash directly. This change corrects two problems:

1. The documentation still recommends the signature algorithm EVP_MD
   names of OpenSSL's legacy APIs. OpenSSL has since moved away from
   thosee, which is why ECDSA was strangely inconsistent. (This is why
   "ecdsa-with-SHA256" was missing.)

2. Node_SignFinal copied some code from EVP_SignFinal's internals. This
   is problematic for OpenSSL 1.1.0 and is missing a critical check
   that prevents pkey->pkey.ptr from being cast to the wrong type.

To resolve this, remove the non-EVP_PKEY_sign codepath. This codepath is
no longer necessary. PR #11705's verify half was already assuming all
EVP_PKEYs supported EVP_PKEY_sign and EVP_PKEY_verify. Also, in the
documentation, point users towards using hash function names which...